The present invention relates to a digital demodulator used with a digital broadcasting receiver for receiving BS digital broadcasting programs, and more particularly to a digital demodulator used with a digital broadcasting receiver for receiving time-multiplexed waves which were digitally modulated by a plurality of modulation methods having different necessary C/N (a ratio of carrier power to noise power) values.
In a BS digital broadcasting method, a hierarchical transmission method of repetitively transmitting, one frame after another, time-multiplexed digital main signals which were modulated by a plurality of modulation methods having different necessary C/N values, such as 8PSK modulated waves, QPSK modulated waves, and BPSK modulated waves, in addition to burst symbol signals inserted in the main signals and capable of being received at a low C/N value. The burst symbol signal is a signal which was BPSK modulated by a known PN code.
In such a hierarchical transmission method, a frame sync pattern and a superframe identification signal have predetermined patterns which were BPSK modulated. A digital demodulator of a digital broadcasting receiver performs absolute phasing which makes the reception phase be coincident with the transmission phase, in order to decode the demodulated baseband signal by a decoder or to perform other operations. In the hierarchical transmission method, a frame sync signal, a TMCC signal for transmission and multiplexing configuration control to be described later and a burst symbol signal are BPSK demodulated, and the absolute phasing is performed in accordance with the reception phase of the received frame sync pattern (absolute phase reception, inverse phase reception).
However, a necessary circuit area of an integrated digital demodulator becomes large because of the presence of an absolute phasing circuit.
An object of the invention is to provide a digital demodulator which does not require an absolute phasing circuit.
The invention provides a digital demodulator for a digital broadcasting receiver for receiving digital modulated waves multiplexed in time axis by a plurality of modulation methods, comprising:
inverter means for selectively inverting demodulated baseband signals in accordance with a BPSK signal having a known pattern prepared in advance; and
carrier reproduction phase error detector means provided with a phase error table having a phase converging point as an absolute phase, the carrier reproduction phase error detector means outputting a phase error signal corresponding to a phase difference between a phase obtained from a signal point of the demodulated baseband signals output from the inverter means and a phase of the phase converging point,
wherein carrier reproduction is executed by controlling a frequency of a reproduction carrier so as to make the phase of the signal point become coincident with the phase converging point.
According to the digital demodulator of this invention, the demodulated baseband signals are selectively inverted in accordance with the phase of the BPSK signal having the known pattern prepared in advance. Therefore, the reference point of the demodulated baseband signals selectively inverted takes an absolute phase. The reference point of the demodulated baseband signals not inverted takes also the absolute phase. The carrier reproduction phase error detector means refers to the phase error table, and outputs a phase error signal corresponding to a phase difference between the phase obtained from a signal point of the demodulated baseband signals selectively inverted and the phase of the phase converging point of the absolute phase. The carrier reproduction is executed by controlling the frequency of the reproduction carrier so as to make the phase of the signal point become coincident with the phase converging point. The phase point of the reception signal is therefore converged to the absolute phase and the reception signal is subjected to the absolute phasing. An absolute phasing circuit is therefore unnecessary.
The digital demodulator of this invention further comprises the carrier filter of a low-pass filter, the carrier filter being input with the phase error signal from the carrier reproduction phase error detector means and stopping a filtering operation during a TMCC section, and a BPSK signal section, a QPSK signal section and an QPSK signal section of main signals, and the carrier reproduction being executed in accordance with an output of the carrier filter.
The digital demodulator of this invention is provided with the carrier filter of a low-pass filter whose filtering operation stops during the TMCC section, and the BPSK signal section, a QPSK signal section and an 8PSK signal section of the main signals. Although the phases of the demodulated baseband signals during the BPSK signal section, QPSK signal section and 8PSK signal section of the main signals are compared with the absolute phase, the filtering operation of the carrier filter stops during these signal sections and any practical problem will not occur.